Question

I’ve been studying magic methods in Python, and have been wondering if there’s a way to outline the specific action of:

a = MyClass(*params).method()

versus:

MyClass(*params).method()

In the sense that, perhaps, I may want to return a list that has been split on the '\n' character, versus dumping the raw list into the variable a that keeps the '\n' intact.

Is there a way to ask Python if its next action is about to return a value to a variable, and change action, if that’s the case? I was thinking:

class MyClass(object):
    def __init__(params):
        self.end = self.method(*params)

    def __asgn__(self):
        return self.method(*params).split('\n')

    def __str__(self):
        """this is the fallback if __asgn__ is not called"""
        return self.method(*params)

Answer 1

There should be no difference between calling MyClass(*params).method() directly and assigning it to a variable. What you may be seeing here is your interpreter automatically printing return results, which is why it appears to be split while the variable value contains EOL markers.

There is no way to override default assignment to a variable. However, by using an object, you can easily provide your own hooks:

class Assigner(object):
    def __init__(self, assignment_callback):
        self.assignment = assignment_callback

    def __setattr__(self, key, value):
        if hasattr(self, 'assignment'):
            value = self.assignment(value)
        super(Assigner, self).__setattr__( key, value )       

def uppercase(value):
    # example function to perform on each attribute assignment
    return value.upper()

Then in your code, rather than assigning to a variable directly you assign to attributes on your object:

>>> my = Assigner(uppercase)
>>> my.a = 'foo'
>>> print my.a
FOO